Hacker is a term applied often to computer software or computer hardware programmers, designers and administrators, and especially those who are perceived as experts or highly accomplished. A hacker is also someone who modifies electronics, for example, ham radio transceivers, printers or even home sprinkler systems for extra functionality or performance. The term usually bears strong connotations, but may be either favorable or denigrating depending on cultural context.

In computer programming, a hacker is a software designer and programmer who builds programs and systems that garner the respect of one's peers. A hacker can also be a programmer who hacks or reaches a goal by employing a series of modifications to exploit or extend existing code or resources. For some, "hacker" has a negative connotation and refers to a person who "hacks" or uses kludges to accomplish programming tasks that are ugly, inelegant, and inefficient. This pejorative form of the noun "hack" is even used among users of the positive sense of "hacker" (some argue that it should not be due to this pejorative meaning, but others argue that some kludges can, for all their ugliness and imperfection, still have "hack value").
In computer security, a hacker is a person who specializes in work with the security mechanisms for computer and network systems. While including those who endeavor to strengthen such mechanisms, it is more often used by the mass media and popular culture to refer to those who seek access despite them.
In other technical fields, hacker is extended to mean a person who makes things work beyond perceived limits through their own technical skill, such as a hardware hacker, or reality hacker.
In hacker culture, a hacker is a person who has attained the respect of one's peers.